1. Field of the Disclosure
The present invention relates to the field of photocrosslinkable writing inks intended to be used as a security ink.
A pigment-based ink can be used as a security ink, i.e. an ink that cannot be erased and thus cannot be forged. This is because pigments are, by definition, insoluble in solvents. Thus, when deposited on a sheet of paper (by means of a writing instrument), they infiltrate into the fibers of the paper, by means of the binder, and remain “trapped” therein. It is thus very difficult to dislodge them therefrom, even by means of solvents in which the paper on which the deposit has been made has been soaked.
However, some of these inks can be removed from the paper by abrasion or friction and are thus not always permanent.
Furthermore, since the pigments are in dispersion in the medium under consideration, the work carried out upstream to formulate the ink is more complex than in the case of a dye-based ink. Indeed, the dispersion requires the homogenous milling of the pigments and the obtaining of particles having a size of less than one micron in order to obtain a homogeneous dispersion (without deposit or segregation). In addition, since the application envisaged is that of a writing ink, the size of the pigments must not be greater than the size of the channels of the writing tip, so as not to block them, which would otherwise inevitably lead to the writing being irreversibly stopped. Furthermore, in addition to the stabilization during the dispersion phase (absence of deposit or of segregation), a long-term (>3 years) stabilization is required for this application.
Conversely, an ink containing dyes is very easy to formulate: the dyes are soluble in the medium, and if there are no additional additives in solid form, the problems previously mentioned for pigment-based inks will not be encountered.
On the other hand, dyes pose the following problems: firstly, they have a weak resistant to light (in certain cases), but especially, after drying of the deposits on the paper, they can be re-dissolved by simply soaking the mark deposited on the paper in the appropriate solvent. Furthermore, in addition to the fact of dissolving these dyes, some very aggressive solvents break the chromophore bonds, thus destroying the color. Thus, it is possible to completely remove the line and, under these conditions, a dye-based ink cannot be used to make a security ink therefrom.
The inventors have surprisingly noticed that it is possible to use dye-based inks as a security writing ink by trapping the dyes after deposit on the paper by means of a chemical reaction, photocrosslinking, which traps the dyes in a network resistant to the conventional panel of solvents (standardized by the Laboratoire National de métrologie et des Essais (LNE) [French National Laboratory for metrology and tests] on the basis of standard ISO 12757-2), which makes it possible to give the ink an indelible nature.
In particular, the inventors have discovered that it is possible to use photocrosslinkable inks in writing instruments. This type of ink has the advantage of rapidly crosslinking, thereby making it possible to rapidly confer on the ink its security ink nature. Indeed, after application of the ink to the paper, it is irradiated by a UV or visible source which causes crosslinking of the ink. The dyes are therefore trapped inside the chemical network formed within the paper fibers themselves. This very rapid reaction makes it possible to “protect” the dyes, and thus the ink line formed cannot become erased (without damaging the paper), whatever the solvent used. Furthermore, it also does not become erased by mechanical action (friction).
In addition, the inventors have been able to formulate these inks without solvent, which makes it possible to avoid all the ecological problems encountered with organic solvents and all the slow-drying problems encountered with aqueous solvents.
2. Description of the Related Art
Photocrosslinkable printing inks are known from applications WO 2008/093071 and WO 2008/009987. However, these inks are not usable in writing instruments of ballpoint pen type, in particular ballpoint pens with a flow rate of between 10 and 300 mg/200 m.
Indeed, for example in the context of application WO 2008/093071, the ink described is an inkjet printing ink of which the viscosity must be less than 100 mPa·s at 25° C. and the temperature at which it is used is generally 40° C. Such a viscosity makes it unsuitable for use in ballpoint pens: it is too fluid and the ink would leak via the point of the pen. In addition, the UV power to be applied in order to obtain efficient crosslinking is very high (use of 2X80W/cm lamps), which is not compatible with consumer safety for writing instruments. In the context of application WO 2008/009987, the printing inks described are “screen printing” inks. They are inks which have a very high viscosity, of between 20 and 80 Pa·s at 25° C. However, viscosities above 30 Pa·s are not easy to use in the context of ballpoint pens since they require specific writing systems with pressurization of the cartridge. Moreover, in application WO 2008/009987, the pigment content is very high (between 20% and 60% by weight and even preferably between 40% and 50% by weight). Such an amount of pigment is impossible to use in a ballpoint pen ink since the viscosity obtained would be too high and there would be a high risk of decanting and of blocking of the point by the pigments.
Application WO 2006/111494 describes a photocrosslinkable ink, optionally usable in ballpoint pens. However, no example of producing an ink for a ballpoint pen is present in said document. The production examples relate instead to the printing inks of which the viscosity is not suitable for use in pens, in particular ballpoint pens. In addition, this ink recommends the presence of a stabilizer consisting of a mixture of sterically hindered nitroxyl radical and of quinone methide. Finally, said document does not specify whether the ink described is a solvent-free ink.